This invention relates to marine seismic sources and more particularly to such sources in which a slug of liquid is propelled into surrounding water by an expanding volume of pressurized gas.
In seismic surveying in a body of water, air guns submerged in the water are a common means for generating seismic energy to investigate geological conditions and formations in the earth below or adjacent to the body of water. For this purpose, one or more of such air guns are submerged in the water; compressed air, or other gas or gases under pressure, is fed to the submerged guns and temporarily stored therein. At the desired instant, the air guns, the seismic energy sources, are actuated, i.e., fired, and pressurized gas, usually highly compressed air, is abruptly released into the surrounding water. In this manner powerful acoustical waves are generated, and the waves are capable of penetrating deeply into the subsurface material of the earth to be reflected and refracted therein by the various strata and formations. The reflected or refracted acoustical waves are sensed and recorded to provide information and data about the geological conditions and formations.
In order to avoid the generation of strong secondary impulses, such as can be created by the oscillating bubble of discharged air from an individual air gun, seismic energy sources known as "water guns" have recently been used. One such water gun is discussed in an article by P. Newman in the Oil and Gas Journal, Aug. 7, 1978, Pages 138-150. In that water gun, water is pushed from the gun by a piston propelled by expanding pressurized gas. After such a water gun is fired, the expanded pressurized gas is vented into the surrounding water over a relatively long period of time such that the discharged air does not generate any significant seismic signal which would interfere with the seismic signal generated by the propelled water. The water gun is then reset, and it is recharged by refilling the gun with pressurized air.
Both air guns and known water guns require that the gun be recharged with pressurized air (or other pressurized gas) after each firing (shot) of the gun, since the source of the energy for propelling the air or water is pressurized air which is recharged into the gun from a compressed air supply on shipboard. With each shot, or firing of the gun, a typical air gun requires the discharge of a relatively large number of cubic inches of air pressurized to for example 2,000 to 4,000 pounds per square inch. The size of the firing chambers of air guns of various sizes may range in volume upwards to several thousand cubic inches. When an array of, for example, ten to forty air guns is fired once every few seconds, the volume of air which must be compressed by the high pressure air compressors on shipboard is quite large. For example, a typical array of thirty air guns in use today may require that approximately one thousand cubic feet of air at atmospheric pressure be compressed per minute of operation during a seismic survey. To provide such a great volume of high pressure air, large air compressors are required. Such compressors occupy a substantial portion of the very limited amount of the valuable space available on board ship, and they require considerable amounts of energy to power them.
Even water guns require that a substantial amount of air be compressed to high pressures on board ship. In such seismic sources, the expanded pressurized gas must be vented into the water in order to reset the water gun. Thus, substantially the same large volume of compressed air is consumed as in the case of air guns of similar size. The above-identified article by P. Newman is entitled Water Gun Fills Marine Seismic Gap, and it compares the operation of a water gun with an air gun. The author states: "Operation is very similar to that of a standard air gun, in fact identical electrical firing circuits and compressed air supplies are commonly used." This statement confirms that substantially the same large volume of compressed air is consumed by water guns as by air guns of similar size.